Forces and Free-Body Diagrams

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• Forces Review• Common Forces• System Diagrams and

Free-Body Diagrams• Analyzing Forces• Force Vectors• Choosing a coordinate

system• Practice

Forces around us

• What happens when forces are in equilibrium?

Force Review

• A force is a push or pull.

• Unbalanced forces result in the acceleration, compression, stretching, or twisting of objects.

Common ForcesForce Symbol FunctionGravity Fg Force of attraction between all objects

Normal FNForce perpendicular to the surfaces of objects in

contact

Tension FTForce exerted by materials, such as ropes, fibres,

springs, and cables, that can be stretched

Friction FfForce that resists motion or attempted motion

between objects in contact

Kinetic Friction FK Force that acts against an object’s motion

Static Friction FSForce that tends to prevent a stationary object from

starting to move

Air Resistance FairFrictional force that opposes an object’s motion

through air

Applied Force FappAny contact force not previous included

System Diagrams and FBD

• A system diagram is a sketch of all of the objects involved in a situation.

• A Free Body Diagram (FBD) is a drawing of a single object that shows only the forces (as vectors) acting on that object.– Always indicate the +x and +y directions

System Diagrams vs. FBD

Check Your Understanding

• You toss a ball vertically upward. Draw an FBD of the ball just before it leaves your hand.

Check Your Understanding

• A child is pushing with a horizontal force against a chair that remains stationary. Draw a FBD of the chair.

Check Your Understanding

• A child pulls a sleigh up a snow-covered hill at a constant velocity with a force parallel to the hillside. Draw a FBD of the sleigh.

Analyzing Forces

• When analyzing a problem involving forces acting on an object, you must find the sum of all the forces acting on that object (called the net force)

FFnet

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Force Vectors

• In order to deal with force vectors that point in odd directions, we will resolve the vectors into components of the coordinate system.

wagon

F

Fx

Fy

Choosing a coordinate system

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Check Your Understanding

• In hitting a volleyball, a player applies an average force of 9.9 N [33o above the horizontal] for 5.0 ms. The force of gravity on the ball is 2.6 N [down]. Determine the net force on the ball as it is being struck.

Check Your Understanding Solutions

Check Your Understanding• A boat is secured to a lakeside pier with two horizontal

ropes. A wind is blowing offshore. The tensions in the ropes are F1 = 48 N [E 16° N] and F2 = 48 N [E 16° S].

(a) Use a vector scale diagram to determine the sum of the tension forces in the two ropes. (b) assuming that the net horizontal force on the boat is zero, determine the force of the wind on the boat.

Check Your Understanding Solutions

a) = 48Cos16 = 46.1 x 2 = 92 N [E]

b) If the tension force is 92 N [E] and the boat is stationary, the force of the wind must be 92 N [W]

PracticeDetermine the net force on the following objects.

1) At a particular instant, a soaring bird travelling east is subject to an upward lift of 3.74 N, a downward gravitational force of 3.27 N, and a horizontal air resistance force of 0.354 N west.

2) A long-jump contestant experiences at the instant of landing a gravitational force of 538 N [down] and a force, applied by the ground to the feet, of 6382 N [28.3° above the horizontal].

3) A crate is being dragged across a horizontal icy sidewalk by two people pulling horizontally on cords (below). The net horizontal force on the crate is 56 N [16° S of E]. The tension in cord 1 is 27 N [E]. If friction is negligible, determine the tension in cord 2.

Practice Answers

1) Fnet = 0.588 N [W 53.0° N]

2) Fnet = 6.15 x 103 N [E 23.9° N]

3) T2 = 31 N [E 30° S]